December 1, 1893.] 



KNOWLEDGE 



229 



SHOOTING STARS AND THEIR TRAILS. 



By A. C. Ranyard. 



ON any clear moonless niglit cue never need wait 

 many minutes to observe a meteor darting across 

 the sky. Those most frequently seen appear and 

 disappear almost before the eye has had time to 

 note their colour and the direction of their motion ; 

 but occasionally larger meteors are observed which gene- 

 rally move more slowly across the heavens, and sometimes 

 leave a luminous streak in their wake. 



Prof. jNlax Wolf, of Heidelberg, has on several occasions 

 succeeded in obtaining photographic traces of meteors 

 which have appeared to the eye as large as stars of the 

 first or second magnitude. The photograph which he has 

 kindly sent for reproduction in this month's number of 

 Knowledge shows two of such meteor tracks. They cannot 

 both belong to the same meteor family, for their paths 

 cross, and they therefore cannot have diverged from a 

 common radiant area, as all meteors travelling parallel to 

 one another in space appear to do. 



The rich groups of meteors which the earth encounters 

 at various times in the year do not appear to diverge 

 accurately from a point in the heavens. Eye observations 

 have shown that they diverge from a radiant region of 

 sensible area ; for though the meteors are, no doubt, 

 moving absolutely parallel to one another before they 

 meet the earth, they are slightly deflected from their 

 original course after entering the earth's atmosphere, prob- 

 ably by reason of the irregular shapes of many of the 

 meteoric masses, which causes them to skid on one side, 

 as a stone or shell thrown into the sea is deflected by the 

 resistance of the water. Sometimes meteor paths show a 

 very sensible curvature, but in the case of eye observations 

 there is always some difficulty in accurately noting small 

 deviations from straightness. 



Prof. Max Wolf's photographs enable us to detect the 

 curvature of meteor paths with great precision. If a 

 straight-edge be put against the smaller meteor track on 

 our plate, viz., that wliicli passes horizontally across the 

 page, it will be seen that the path is decidedly convex 

 towards the north, an effect which cannot be due to the 

 distortion of the lens with which the photograph was 

 taken ; for, as will be seen by the distorted images of the 

 stars which lie on the southern and eastern sides of our 

 plate, it is an enlargement from only a part of the original 

 negative, and the centre of the original negative was some- 

 where towards the top right-hand corner of our plate, 

 where the images of the stars are round. Consequently, any 

 distortion caused by the short-focussed photographic lens, 

 with which the original negative was taken, would have 

 given a still greater curvature to the other meteor track 

 which passes diagonally across the page. The lenses 

 which have been used for enlarging the picture had much 

 longer foci, and the distortion caused by them may be 

 neglected. 



The meteor which has passed horizontally across the 

 plate has drawn a clear sharp line of even thickness from 

 side to side of the picture ; but the larger meteor, which 

 transited the field diagonally, has left a photographic 

 trace which is decidedly thicker in some parts than in 

 others, an appearance which was probably caused by 

 differences of brightness at different parts of its path. 

 The track of this larger meteor, when examined by a lens, 

 is seen to break up, in some parts of its course, into knots 

 or nuclei, separated by small intervals, as if the variations 

 of brightness were very rapid. 



It is diificult to conceive of such rapid alterations in 

 the brightness in an ordinary incandescent solid such as 



we are able to experiment upon in the laboratory, but in 

 tlie case of a meteorite we have a cold body witii an 

 incandescent skin or film of boiling material on its surface, 

 for the meteor has just arrived from the cold of space and 

 is being heated by the vigorous bombardment of its surface. 

 The sudden expansion of the surface material, or of the skin 

 beneath the boiling film, must give rise to fierce strains, 

 which, in the case of a large bolide, sometimes rend it 

 with a fearful noise like loud thunder, which has been 

 heard for a distance of fifty miles, although the explosion 

 or rending of the stone has taken place in the thin upper 

 air. 



Probably under no other conditions is a stone subjected 

 to such fierce strains as when it plunges with planetary 

 velocity into an atmosphere and is rapidly heated from its 

 outside. No doubt, a soft stone would quickly crumble to 

 pieces on being subjected to such an ordeal, and with 

 every fragment detached the surface of the moving mass 

 would be increased, and the area of incandescent surface, 

 and consequently the brightness of the meteor would be 

 proportionately increased. 



The uneven photographic trail of this little meteor tells 

 an eloquent tale as to the rapidity with which the crumb- 

 ling fragments from the moving mass are driven into 

 vapour and lost to sight. In the case of larger meteors 

 the moving body is sometimes surrounded by an envelope 

 of incandescent vapour, many hundred yards, and some- 

 times several miles, in diameter. Thus a remarkable fire- 

 ball, which was well observed in the south-west of England 

 on the Gth of November, 1869, was during its passage 

 through the air surrounded by a luminous cloud more than 

 four miles in diameter. It first became visible in the air 

 at a point about ninety miles vertically above Frome, 

 in Somersetshire, and disappeared at a height of about 

 twenty-seven miles above the sea near St. Ives, in Cornwall, 

 having travelled a distance of one hundred and seventy 

 miles in about five seconds, with an average velocity of 

 about thirty-four miles a second. This meteor was entirely 

 driven into vapour, and after the fire-ball had disappeared 

 a luminous streak or cloud about fifty miles long and four 

 miles in breadth remained visible against the sky for some 

 fifty minutes. 



The heat developed at the surface of a body moving 

 through a gas is proportional to the square of the velocity 

 of motion. A body moviEg with a velocity of about a 

 mile a second would have its surface raised to a tempera- 

 ture of about red heat, and with a velocity of only ten 

 miles a second the surface of the moving body would be 

 raised to a temperature of white incandescence far above 

 that necessary to melt and vaporize all known substances. 

 As Prof. Young has remarked, the temperature of the 

 surface of a moving body is independent of the density of the 

 air through which it is moving ; but the velocity lost and 

 the quantity of heat developed in a given time is, of course, 

 greater where the air is dense, and the quantity of air 

 which strikes the surface of the moving body, and is 

 rendered incandescent by the motion imparted to it, 

 evidently increases with the density of the air through 

 which the moving body is rushing. 



It follows from this reasoning that the surface of a 

 meteorite must be raised to its highest incandescence 

 immediately it plunges into the thumest layers of the 

 upper atmosphere ; but certainly, as a general rule, meteors 

 mcrease in brightness the deeper they plunge into the 

 earth's atmosphere. We are therefore forced to conclude 

 that the light given by meteors is chiefly derived from the 

 incandescent air and vapours surrounding them, and not 

 from the incandescent surface of the solid mass, or the 

 liquid film surrounding it, which is so rapidly boiling 



